1. Field of the Invention
The present invention relates to a cleaning unit for a photoreceptor in a xerographic printing machine, or the like.
2. Description of the Related Art
FIGS. 5 and 6 are sectional views showing a conventional cleaning unit for a xerographic printing machine. Toner 2, left on photoreceptor 1 after it is turned through a sequence of process steps consisting of charging, exposure, development, and fuzing, is electrostatically removed by conductive cleaning brush 3, which is electrically biased by biasing voltage supply 8. Toner 2 is then electrostatically transferred to cleaning roll 4 which is also electrically biased by another biasing voltage supply 9 of which the bias voltage is higher than that of biasing voltage 8. Subsequently, toner 2 is transferred to rubber blade 11, which is fixedly held by blade holder 5. Toner 2 is scraped off cleaning roll 4 by rubber blade 11 and discharged outside the printing machine by toner collecting screw 7.
Of course, when the top or cutting edge of rubber blade 11 is worn or cracked, it does not properly scrape toner 2 off of cleaning roll 4. In such a case, toner will stick to conductive cleaning brush 3 and penetrate into the bristles thereof so as to fill conductive cleaning brush 3 with toner 2. Conductive cleaning brush 3, when filled with toner 2, may fail to remove toner 2 from photoreceptor 1, and consequently, the printer fails to print properly.
In the conventional cleaning unit, rubber blade 11 is made of urethane rubber, for example, and cleaning roll 4 is made of metal, such as aluminum or stainless steel. To remove toner 2 from the surface of cleaning roll 4, rubber blade 11 is resiliently pressed against cleaning roll 4. Such a cleaning unit, however, has the following defects:
1) The lifetime of the blade top is short, since it is easily worn. Thus, the blade must be frequently replaced.
2) The blade top is readily damaged by hard foreign materials, such as carriers and paper dust.
3) Frictional resistance F between the blade and the cleaning roll is large. Therefore, the blade tends to roll up at the cutting edge.
4) Because of factors (1) and (3) above, the pressure N of the blade against the cleaning roll is limited. Because the blade is resilient, the foreign materials are readily introduced between the blade and the cleaning roll. This results in white lines on the reproduction.
5) The toner is prone to stick to the surface of the cleaning roll.